Tolerance Stack Question 1

[pmarc]

I just need a double check on following tolerance stack calculation. What would be the minimum and maximum distance between bottom surface of the part (datum feature C) and hole surface using worst-case approach?

 

[Belanger]

To have the geometric tolerance control the location of the holes from the edges, you need to have datum letters (representing the edges) be mentioned at the end of the GD&T box. I'd recommend the back face of the plate as a datum too ... call that datum A. Then have the bottom edge labeled as datum B, and the left edge as datum C. Then have A, B, and C given after the 0.25 section of the GD&T callout. The 15.15 would stay basic, but now your 4 mm dimension would also be basic (and drop the ±0.25).

Consult any simple GD&T reference book/guide for an example of this scenario.

Your answer for "X" would then be a max of 4.125 and a min of 3.875, because the given geometric tolerance is diametrical (think of the tolerance zone as being centered exactly at 4.00 so it translates to .125 in any radial direction). The max distance between the holes would be 39.25 and the min would be 38.75.



John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[pmarc]

First of all I would suggest not to use coordinate dimensioning (4[±]0.25) for locating left hole from the left edge of the part. I would recommend changing it to basic dimension, assigning three mutually perpendicular surfaces as datum features A, B, C (like in my original example) and put position tolerance to the pattern of two holes with the reference to A, B, C. This would at least eliminate ambiguity of current dimensioning scheme.

I do not think you want to dimension arc of the hole from the edge of the plate. In most of similar cases position tolerance is applied to control location of holes axes relative to other features.

Why the hole position (Ø 0.25) has no impact on dimension (X)? Because you directly defined that the distance between axis of left hole and left surface of the part is 4[±]0.25. With your dimensioning scheme position tolerance is important only when any relationship between both holes is considered.

 

[drawoh]

...
drawoh, How did you get the positional tolerance Ø0.3mm from the hole at Ø15.2mm.
If you could explain i would be very gratefull.

The hole has a positional tolerance of 0.1mm diameter at maximum material condition. It applies at the minimum diameter of 15.0mm. As you increase the actual diameter, you increase the positional tolerance. At 15.2mm diameter, the positional tolerance is 0.3mm.

JHG

 

[BLUEHONDA]

Could anybody show me how i would work out what the smallest amount of material would be left between the hole and the outside edge of the pad.
Several people have had a go at this and given me different answers.
Is there a quick and easy method of working this out? And please bare in mind that I'm very new to this stuff, so explanation for dummies please.

Thank you all.

 

[pmarc]

BLUEHONDA,

Look to attached presentation. It is showing an example of how you could have thought about the stack I asked for a double check at the very beginning of the post. I tried to explain it as simply as I could. Hope this helps.

http://files.engineering.com/getfil...ae-9e71-85710eb4d765&file=Tolerance_stack.pdf

P.S.: If you are asking for a quick and easy method of dealing with tolerance stacks, I would say it really depends on one's experience, preferences and how complicated a stack is. The method I like to use (it is not the one I showed in the attachment) is really easy and consistent for me, but that does not mean it would be clear for you. First of all try to do as many stacks as possible, choose different calculation methods as described in different GD&T books (e.g. Krulikowski, Fisher or other authors) and you will quickly notice which one is most suitable for you. Additional Tolerance Stack-up training would also help for sure as you you seem to be at a beginner level.

 

[SeasonLee]

Excellent step by step explanation, pmarc.

SeasonLee

 

[ak762]

pmarc
Am I correct understood from your PostScriptum that stackup results can be differ and it's depend from calculation methods (e.g. Krulikowski, Fisher or other authors)?

 

[Belanger]

No, the answers should be the same. He's saying that you may prefer one method over another.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems